If the seller is not set up to take credit cards (which often needs a hefty deposit up here) then I don't think there is an easy answer. PayPal was created in part to deal with situations like that -- buyer using cc but seller not set up for them.
My rule is don't buy used unless one of the following is true :
- the price is good enough and low enough that if I get burned I don't really care
- I know where the seller lives and could drive over there with a bunch of beer-soaked buddies one night, and THEY KNOW IT